Behavioral Responses of Sea Turtles to Light
Sticks Used in Longline Fisheries

During the past several decades, many sea turtle populations have declined
significantly. In particular, nesting populations of leatherback (Dermochelys
coriacea) and loggerhead (Caretta caretta) turtles in the Pacific
Ocean have dramatically decreased. Turtle bycatch associated with pelagic
longline fisheries has been implicated as one factor contributing to this
decline in at least some populations.

Pelagic longline fishing involves a
single main fishing line that can stretch over 50 km with thousands of
individually hooked lines branching off from the main line. This fishing method
is used in every ocean basin and commonly targets tunas,
swordfish, and dolphin fish.
In addition to these targeted fish, however, longline fisheries also
inadvertently catch sea turtles.

Loggerhead
and leatherback turtles are the species that most commonly come in contact with longlines. Turtles are often hooked in the mouth, throat, or digestive tract
and subsequently drown when they are unable to surface to breathe. Turtles are
also sometimes hooked in their flippers and carapaces, or become entangled in
the lines.

A sea turtle with a longline fishing hook embedded in its mouth.

Strategies to diminish the impact of
longline fisheries on sea turtle populations have included seasonal and area
fishing closures, attempts to decrease mortality of captured turtles through
better handling practices, alteration of fishing methods, and changes in gear.
In the North Atlantic swordfish fisheries, a reduction of sea turtle bycatch was
achieved by using a special kind of hook with a circular shape.

An understanding of the sensory
stimuli that attract turtles into the vicinity of longline sets may be useful in
developing additional gear modifications that will reduce turtle bycatch. A
common practice in some longline fisheries is to attach glowing lightsticks to
the branch lines in order to attract fish into the vicinity of the baited
hooks. Lights used include chemiluminescent lightsticks and battery-powered
light-emitting diodes (LEDs). Although these lights are intended to
attract fish, the possibility exists that they also attract sea turtles.

Several types and colors of glowing light sticks are used in longline
fisheries to attract fish.

To
investigate whether lightsticks used in longline fisheries attract turtles, we
conducted experiments using an orientation arena and tracking system originally designed for studies on
magnetic orientation of sea
turtles. Juvenile loggerhead turtles were placed into a soft cloth harness and tethered to
an electronic tracking system that monitored the direction that each turtle swam
toward. Some turtles were tested with glowing lightsticks in the arena,
whereas others were tested in the same tank in darkness as a control.

Results and Interpretation: The turtles consistently swam
towards glowing light sticks of all colors and types. In contrast, they
ignored lightsticks that were not illuminated. These findings imply that
turtles are attracted to the illumination produced by lightsticks and suggest
that such lights may be one factor that draws turtles into the vicinity of
longlines. An important caveat, however, is that these experiments were
carried out in the laboratory. Thus, additional studies under more natural
conditions in the ocean will be needed to confirm the findings.

About the Research: The study described on this web page
is based on the following published paper:

The lead researcher on this
study was Dr. John Wang, a former graduate student and postdoc in the
Lohmann Lab who
is presently a postdoctoral fellow with the University of Hawaii and the
National Marine Fisheries Service.